Field of the Invention
The present invention relates to the field of sodium/potassium/sulfur rechargeable batteries.
Related Art
Elemental sulfur has the highest theoretical capacity density (1675 mA h g−1) and the lowest cost of all known cathode materials for lithium batteries. Previous reports have shown that a sodium/sulfur (Na/S) battery demonstrates significant advantages such as high energy density (theoretical specific energy density of 760 W h kg−1), low cost material (abundance of sulfur and sodium in nature), and low rate of self-discharge and high power density. A typical Na/S battery consists of sulfur at a positive electrode and sodium at a negative electrode separated by a solid beta alumina ceramic electrolyte. However, this type of Na/S battery must be operated at approximately 300° C. to ensure sufficient Na+ conductivity in the electrolyte. At this operation temperature, both sulfur and sodium electrodes are in the liquid (molten) state. Thus, taking into consideration the extensive and even potentially explosive reactions between liquid sulfur and liquid sodium, safety is an issue for such high temperature Na/S batteries. In addition, active cathode materials (sulfur and sodium polysulfide) are corrosive and are considered to be one of the major failure mechanisms for Na/S batteries.
Another additional problem that has been previously reported is that in a Na/solid beta alumina ceramic (b-Al2O3) electrolyte/S battery, Na2S2 solid will jam the ionic channel of the solid b-Al2O3 electrolyte and the discharge process will be ended before the formation of Na2S2. As a result, the specific capacity of sulfur is actually less than 836 mA h g−1 for a Na/b-Al2O3/S battery. Nevertheless, a room temperature design that utilizes liquid sodium as an anode remains to be an attractive solution.
Several groups are developing room temperature sodium ion batteries which are promising substitutes for lithium ion batteries in various application areas. However, in a low temperature Na/S battery, the sulfur cathode will encounter the same problems as Li/S batteries, i.e., low utilization of active material, poor rechargeability and dissolution of polysulfides into the electrolyte.
Previous studies describe sulfur composite cathode materials with sulfur embedded in a PEO polymer matrix, which exhibited good electrochemical performances in lithium batteries.
Additional studies report that sulfur composite materials were used as cathodes for room-temperature Na/S battery with a liquid electrolyte. Sodium/sulfur (Na/S) batteries were assembled with a sodium metal anode, liquid electrolyte and a sulfur composite cathode. Their electrochemical characteristics have been investigated at room temperature. Their charge/discharge curves indicate that sodium can reversibly react with sulfur at room temperature. The specific capacity of the sulfur composite cathode material in the first cycle was initially about 655 mA h g−1 and stayed at about 500 mA h g−1 up to the 18th cycle with about 100% charge/discharge efficiency. However, the same study reports issues with sodium dendrite formation wherein a thick black moss, so-called “sodium dendrite”, covered the surface of the sodium anode. Sulfur composite cathodes could be still well charged and discharged with a fresh sodium anode. A similar phenomenon was observed in previously reported Li/S batteries. This phenomenon indicates that the dendrite is also a critical problem for room temperature Na/S batteries.
Another area of rechargeable battery research involves finding safe ways to improve conductive electrolytes. Solvent-free polymer electrolytes are of immediate interest for rechargeable lithium batteries. This is directly related to safety issues since volatile organic electrolytes (e.g. propylene carbonate) can incinerate in case of malfunction of the Li battery (e.g. thermal run-away). Poly(ethyleneoxide) (PEO), an inert polymer, belongs to the most intensively studied materials for this purpose. The fact that PEO builds complexes with Li salts and displays both thermal as well as interfacial stability makes PEO a promising candidate as polymer electrolyte. However, at room temperature the ionic conductivity of Li salts dissolved in PEO is limited because the highly symmetrical repeating units in PEO tend to crystallize. Crystalline regions in PEO (m.p. approx. 65° C.) are not available for ion transport, and conductivity is therefore limited to the amorphous regions of PEO (glass transition temperature Tg approx. −55° C.).
To address the issue of reducing crystallization in PEO and increase ionic conductivity a study reported that PEO/ionic liquid composites were investigated as solvent-free electrolytes for lithium batteries. Ternary electrolytes based upon PEO, an ionic liquid and a conducting salt were UV cross-linked with benzophenone as a photoinitiator. Crosslinking leads to an increase in mechanical stability of the PEO composites. This straight-forward process provided a way to increase the content of ionic liquid and thus to raise ionic conductivity without loss of mechanical stability. Impedance measurements showed that the ionic conductivity of the composites was not affected by the UV curing process. Moreover, the UV curing process caused a decrease in the degree of crystallinity in the PEO composites which contributed to an increase in ionic conductivity.
What is needed is a re-chargeable battery design that combines the benefits of utilization of optimized components including a non-dendrite forming sodium/potassium liquid metal anode, a sulfur/polyacrylonitrile (PAN) conductive polymer composite cathode, a cross-linked solid polyethyleneoxide (PEO) polymer electrolyte, and a stable interface (SEI) on the solid PEO polymer electrolyte without any of the reported inherent drawbacks associated with prior usage of the individual components in other designs.